Is it acceptable to ask for a modification to a travel to visit graduate school?

I have been accepted to a Ph.D. program in St. Louis, MO and will be visiting the school in late March. I will be reimbursed for flight tickets. The scheduled Open House runs from Thursday to Saturday, and I have been told to book my flights with arrival on Thursday and departure on Saturday. The flight ticket per those specifications is at least $510, while if I arrive on Thursday and depart on Sunday, it would be $50 less. Since I will be committing 5 years of my life to studying there, I would like to know that I like the place, in addition to liking the Department (which I do). So is it okay to ask the secretary if I can book my departure on Sunday and save them $50 (lodging that extra night will be on me)?